After a stoppage or web break in a paper machine or a finishing apparatus for paper, the web has to be made to travel along a path according to the web path geometry again. In order to make a web of several meters in width to travel along a route according to the web path geometry, which can be quite complex in some sections, a tail or a lead-in-strip, which is narrower than the full-width web, is first guided to travel a given distance in the machine direction. There are several auxiliary devices developed for tail transfer, such as suction belts and air blowing devices that convey the strip, and threading ropes or belts between which the leading end of the tail is guided. The transfer distances of such auxiliary devices from the reception point to the delivery point vary in length. As for the numerous methods, examples that can be mentioned include e.g. the rope-assisted threading presented in the international publication WO 97/13032 by the applicant, and the short transfer of the tail implemented by means of a suction belt, disclosed in the U.S. Pat. No. 4,692,215 by the applicant. The Finnish patent 101634 by the applicant, and the corresponding international publication WO 98/38382, in turn, disclose threading through a stack of calender rolls conducted by means of air blowings and suctions. The tail can also be transferred by means of combinations of different devices through a given portion of the machine, i.e. a particular distance along the travel path of the web from the starting point to the end point. As for the different threading methods, reference can be made to the publications U.S. Pat. No. 3,355 349, EP-232 689, U.S. Pat. No. 4,648 942 and EP-332 352.
According to the established practice, the tail is transferred at the running speed of the machine. Thus, on a given portion of the paper machine or finishing machine for paper, a full-width web is passed to a broke processing system at the production speed, and a narrower tail or lead-in-strip is separated e.g. from one edge of the web by means of a suitable cutting method. This tail is transferred in any of the aforementioned ways or by means of a combination of the same, through the next portion at the same running speed, which is at present typically over 20 m/s. When the web has been passed without problems through this portion in which the web path geometry can be quite winding and the tail is passed between several members located close to each other, the web is spread into its full with by means of diagonal cutting.
The significance of a fast threading process is emphasized for example in the U.S. Pat. No. 4,728,396, which discloses threading through a coater. Here, the threading of the web is first conducted at a speed lower than the running speed (production rate), i.e. at a so-called crawling speed, starting from the unwind stand in the inlet end of the machine. However, if a web break takes place, the web is still passed at the production speed to the broke processing system in a point located before the web break point in the travel direction of the web, and a tail is separated from this web and passed through the coater at the running speed, whereafter it is spread into a full-width web travelling at the running speed i.e. at the production speed. This decreases idle time in connection with web breaks, because the speed of the machine does not have to be reduced for threading.
High threading speeds improve efficiency, but at the same time it is necessary to be able to control the tail in threadings in which the web possibly travels even over steep curves due to the winding or meandering nature of the web path geometry, and through narrow points in the travel path, for example between two obstacles located close to each other, of which obstacles one or both can be a rotating roll. Because of this, auxiliary members have been developed which move at a speed lower than the threading speed, and at the same time receive the tail passed at a threading speed corresponding to the running speed while moving to the threading direction. As an example of this can be mentioned the suction tube presented in the international publication WO 98/56701, which receives the tail, at the same time moving to the threading direction at a speed lower than the running speed of the tail. Thus, a special device, which must be moved within the entire threading distance, is required to ensure the travel of the tail.
The Finnish patent 102197 by the applicant, and the corresponding international publication WO 98/54407 also disclose a manner in which it is possible to attain a speed of the tail, which is lower than the running speed without having to change the running speed of the machine portion preceding the threading portion. The idea is based on the mechanical working of the tail in such a manner that its free length is reduced, wherein the threading devices, such as ropes, functioning along the threading portion, can transfer the tail at a speed lower than the running speed. In this method, the reduction of the speed is directly dependent on how well the tail can be shortened for example by wrinkling and the speed has to be adjusted to comply with the change of length of the tail.